Preparing plan ‘b’

7 September 2009

A 3D laser scan of the end faces of the central Tile detector is being used to map its surface in preparation for important detector upgrades in 2013.

The LHC hasn’t got going yet, but there are already plans in the pipeline to, well, replace the pipeline. In 2014, a new layer of Pixel detector – the so-called ‘insertable b-layer’ (IBL) will be installed inside the existing three.

To make room for the new layer, the seven-or-so metres of beam pipe centred on the interaction point in ATLAS will need to be ripped out and replaced with narrower piping. As a 4th pixel layer attached directly onto the new beam pipe, the IBL will improve the performance of the sub-detector. The current innermost layer will by then have received a significant dosage of radiation, owing to its proximity to the beam pipe, so the replacement will act to protect the integrity of the data that the detector reads out.

Inserting a whole new layer of detector into an already packed cavern is no trivial job, especially when the physical reality of complex engineering projects like ATLAS is rarely identical to the initial plans. For this reason, the specifications of the new layer must be subject to ‘reverse engineering’; in other words, they need to be defined with the available space in mind.

Enter Dirk Mergelkuhl, Christian Lasseur, and others from the metrology and beam pipe groups. Together with an external company, they performed a 3D laser scan of the ends of the central Tile Calorimeter earlier this year. The idea is to identify all available space on these surfaces, within a one-metre radius of the beam pipe, where services for the new layer could be added.

The company, a joint venture between HKD Geomatic and Archeotech, provided the laser scanner, which was piloted remotely by Dirk. During the operation, he mounted the machine on a strut or piece of scaffolding with a good angular view of the area. It was able to define 125,000 points each second and, after a few minutes of scanning, Dirk would return and move the scanner to a different angular vantage point. The faces of the Tile were eventually described by a total of 730 million points.

For six months, work has been underway, using a combination of over 30 different reconstruction programs, to try to map the surfaces in question from the clouds of points recorded by Dirk in February. According to Christian, it is not important to define each and every cable, but rather the cable “envelopes” that exist in the region of interest. In fact, it’s the areas with no cables at all that are key: “We’re much more interested by holes, by what doesn’t exist,” he smiles.

Close quarters. Where would you put the extra services?

The team hope to have delivered as much of the final description as possible by the end of the year. “It had to be done now,” says Dirk, joking: “no one wants to be two days next to the beam line after the run period. Everything must be tested and well prepared so that people can just go down, change over, and get out. It needs to work first time round.”

The results of the 3D survey will be used by the engineers who are responsible for planning pathways for the new services and designing tools to insert the b-layer.